The wrist
The wrist is a multipart joint consisting of numerous bones, tendons, ligaments, and muscles that allow the wrist to be stabilized while transmitting force to the hand, but also allow a large range of motion in many directions.
There are two long parallel bones in the forearm (radius and ulna) that run from the elbow to the wrist. The larger bone, the radius, is on the same side as the thumb, and the smaller bone, the ulna, is on the side of the little finger. There are eight small bones (carpal bones) within the wrist.
The thumb and hand
The hand consists of five digits, with the thumb playing a major role. There are large muscles outside of the hand and small muscles inside the hand that—together with the bones, nerves, ligaments, and tendons—allow a wide range of motion but also provide stability to transmit force to the fingers and thumb.
The base thumb joint (CMC joint) is a saddle joint which allows more directions of motion than the fingers. The most important function of the thumb is to oppose the fingers, which allows the manipulation of objects in our environment. The thumb CMC joint is between the first metacarpal bone and one of the small bones in the wrist (trapezium).
Image 1: Wrist/Thumb Anatomy